The present invention relates to a ceramic electronic device and a method of manufacturing the same.
In order to form cavities in ceramic electronic devices according to conventional methods, a paste such as a carbon paste to be decomposed and removed at a temperature up to a ceramic sintering temperature is coated in ceramic plates, or a press plate is compressed on a ceramic green sheet. For example, in order to prepare a conventional laminated ceramic capacitor, a paste containing carbon is coated on a green sheet, and sheets including the sheet with a paste layer were stacked, pressed and sintered to form cavities at the paste portions. Thereafter, a metal such as silver, lead or tin is impregnated in the cavities under pressure to form internal electrodes (Fabrication of Multilayer Ceramic Capacitors by Metal Impregnation; IEEE TRANSACTION ON PARTS, HYBRIDS, AND PACKAGING, VOL. PHP-9, No. 3 September 1973, pp. 144-147).
The first drawback of a piezoelectric electroacoustic transducer device prepared by conventional techniques lies in the fact that a piezoelectric plate, an elastic plate, and a casing are separately prepared and are finally assembled together. In order to assemble such devices in the mass production line, a large number of assembly steps are required. Even if an automated assembly system is used, a considerably high investment capital is required, thus increasing the product cost.
The second drawback of the piezoelectric electroacoustic transducer device is degradation of the performance and reliability caused by adhering the piezoelectric and elastic plates by an adhesive or the like. If a bending resonator is flexed upon reception of an electrical signal or an external sound pressure, a large stress is concentrated in the adhesion layer portion. If the transducer is driven at a large amplitude for a long period of time, the adhesion layer is damaged. In addition, vibration energy is absorbed in the adhesion layer to degrade operational efficiency. Furthermore, the temperature characteristics of the transducer are degraded by an influence of the adhesive.
The third drawback of the piezoelectric electroacoustic transducer device is the limitations of a decrease in thickness of the piezoelectric plate. The resonant frequency of the bending resonator is proportional to its thickness and is in inverse proportion to the square of its diameter. In order to obtain a compact transducer device usable in a low frequency, the thickness of the piezoelectric plate must be minimized. In the process for preparing a thin piezoelectric plate, its thickness is limited, as described above. Even if a thin piezoelectric plate is prepared, it is mechanically damaged when it is adhered to the elastic plate. Therefore, as compared with the case wherein a piezoelectric plate having a relatively large thickness is used, the yield of the piezoelectric plates is greatly decreased.
The fourth drawback of the piezoelectric electroacoustic transducer device is to require fine finish for forming a flat surface of the piezoelectric plate. Upon adhesion of the piezoelectric plate to the elastic plate, if the piezoelectric plate is warped or has a large recess, the plate is damaged and the contact density of the piezoelectric plate adhered to the elastic plate is undesirably decreased. Furthermore, incomplete adhesion also leads to the degradation of energy conversion efficiency of the bending resonator.
In a conventional ink-jet head, a metal press plate is pressed on a green sheet or an acrylic die is set in a green sheet, thereby forming cavities U.S. Pat. No. 4,648,786.
However, according to the conventional cavity or space formation method, satisfactory precision cannot be obtained, or the height of the cavity is undesirably small. In order to form a cavity pattern by screen printing such that a carbon paste to be decomposed and removed at a high temperature is coated on the green sheet, the minimum order is restricted to the formation of a micropattern having a width of 100 .mu.m at a pitch of 200 .mu.m. In this case, the pattern has only a maximum height of 10 .mu.m. Therefore, the resultant cavity is not enough to be used as a cavity for electronic device.
In the conventional case of forming cavities by using a metal press plate or a plastic film press plate, deformation under pressure results in degradation of precision of the pattern.
In particular, if a recess is formed on the green sheet under pressure by using a metal press plate, a ceramic sheet serving as a cover member is stacked thereon and thermally pressed. In this case, part of the overlying green sheet is undesirably inserted in the recess to cause irregular depths of the cavities.
Even if an acrylic sheet is used as a press plate and inserted in the green sheet, the acrylic sheet is deformed, thus degrading the dimensional precision.
In general, the ceramic electronic devices are operated through electrodes. According to a conventional preparation method, it is difficult to form an electrode inside the ceramic body or at a surface portion defining the cavity formed therein. According to the conventional method, after the ceramic sheet is sintered, a thick film is printed or plated to form electrodes on the resultant structure.
According to this method, the electrodes are formed on only the surface of the resultant structure. The use of a piezoelectric ceramic material limits a variety of applications. As a result, it is very difficult to prepare high-performance electronic devices.
In an ink-jet head described in U.S. Pat. No. 4,648,786, ink ejection members such as an ink supply member, an ink channel groove, and an nozzle are made of a ceramic material. In this head, a metal press plate is compressed on a green sheet to form grooves. An acrylic die is pressed and inserted in the green sheet, and another ceramic green sheet serving as a cover member is placed thereon. The stacked sheets are then pressed and sintered. After sintering, a means for generating or applying mechanical vibrations is formed on the upper surface of the sintered body. As shown in FIG. 2, a piezoelectric sheet is used as an upper cover member 010 and sintered. After sintering, driving electrodes 012 and 013 are formed on the both major surfaces of the upper cover member 010.
By using this ceramic body, the upper cover member 010 can be easily adhered to a plate 009 with ink channel grooves. However, the conventional head does not have high precision of ink channels. The nozzle tip is not maintained in the top condition so that the ink ejection is warped.
Grooves for the ink channels are formed by etching, as shown in FIGS. 1(a) and 1(b). In this case, the etching precision has limitations. Variations in nozzle diameter greatly degrading the ink injection characteristics are undesirably increased. In the head structure of FIGS. 1(a) and 1(b), nozzle apertures are constituted by stacking a cover plate 006 on a base plate 001. If misalignment between the plates 006 and 001 occurs at their corresponding edges, ink drops are warped toward a direction of an extended one of the plates 001 and 006. In general, after the base plate 001 is bonded to the cover plate 006, the edge of the nozzle portion is cut or polished to eliminate a step on the adhesion end face. In this case, if a metal material is used to constitute the plates 110 and 006, burrs are inserted in the nozzles by cutting and polishing. Many problems are left unsolved in order to obtain a flat nozzle end face.
When a conventional ceramic material is used, as shown in FIG. 2, it is difficult to form the ink channels with high precision using metal and plastic press plates since the ceramic material is deformed under pressure. In particular, when recesses are formed in the green sheet by using a metal plate, the green sheet serving as the cover member is partially inserted in the recesses under pressure to result in irregular depths of cavities, i.e., the ink channels. Furthermore, even if an acrylic sheet is used and pressed into the green sheet, a large pressure is required to embed the entire acrylic sheet, and the sheet is inevitably deformed to degrade the precision of ink channels.
According to a conventional method, electrodes are formed by printing a thick conductive paste film on the surface after the ceramic material is sintered. The electrodes 012 in FIG. 2 are formed by this method. In order to apply a voltage to this head, corresponding electrodes 013 are required. These electrodes 013 are formed on the inner wall surfaces defining corresponding pressure chambers 011. These electrodes can be formed by electroless plating. With this electrode structure, the wall sandwiched by the electrodes stretches or shrinks upon application of the voltage across the electrodes. Therefore, the wall does not substantially generate a force for ejecting the ink from the pressure chambers. If any, the force is not stable due to bending deformation of the wall. According to the conventional method, therefore, since the electrodes are formed only on the both sides of the pressure chamber wall portions, a sufficient pressure cannot be effectively generated.